The present invention is in the field of battery technology and, more particularly, in the area of additive compounds for use with high-energy electrodes in electrochemical cells.
A liquid electrolyte serves to transport ions between electrodes in a battery. Organic carbonate-based electrolytes are most commonly used in lithium-ion (“Li-ion”) batteries and, more recently, efforts have been made to develop new classes of electrolytes based on sulfones, silanes, and nitriles. Unfortunately, these conventional electrolytes typically often cannot be operated at high voltages and/or at high temperatures. At high voltages, conventional electrolytes can decompose, for example, by catalytic oxidation in the presence of cathode materials, to produce undesirable products that affect both the performance and safety of a battery. Conventional electrolytes may also be degraded by reduction by the anodes when the cells are charged.
As described in more detail below, solvents, salts, or additives have been incorporated into the electrolyte to decompose on the electrode to form a protective film called a solid electrolyte interphase (SEI). Depending on the exact chemical system, this film can be composed of organic or inorganic lithium salts, organic molecules, oligomers, or polymers. Often, several components of the electrolyte are involved in the formation of the SEI (e.g., lithium salt, solvent, and additives). As a result, depending on the rate of decomposition of the different components, the SEI can be more or less homogenous.
In past research, organic compounds containing polymerizable functional groups such as alkenes, furan, thiophene, and pyrrole had been reported to form an SEI on the cathode of lithium ion batteries. See, e.g., Y.-S. Lee et al., Journal of Power Sources 196 (2011) 6997-7001. These additives likely undergo polymerization during cell charging to form passivation films on the electrodes. SEIs are known to contain high molecular weight species. However, in situ polymerization during the initial charge often cannot be controlled in a precise enough manner to prevent non-uniform SEIs comprised of polymer or oligomer mixtures with either heterogeneous molecular weight, heterogeneous composition, or even undesired adducts. The non-uniformity of the SEI often results in poor mechanical and electrochemical stability, which is believed to be a main cause of cycle life degradation in lithium ion batteries. Thus, the improvement in cell performance using these materials is limited.
Further, certain organic polymers have also been used as solid electrolytes for lithium ion batteries due to the generally low volatility and safety of polymeric molecules as compared to smaller organic molecules, such as organic carbonates. However, practical application of such systems has been limited due to poor ionic conductivity.
For high-energy cathode materials, electrolyte stability remains a challenge. Recently, the need for better performance and higher capacity lithium ion secondary batteries used for power sources is dramatically increasing. Lithium transition metal oxides such as LiCoO2 (“LCO”) and LiNi0.33Mn0.33Co0.33O2 (“NMC”) are state-of-the-art high-energy cathode materials used in commercial batteries. Yet only about 50% of the theoretical capacity of LCO or NMC cathodes can be used with stable cycle life. To obtain the higher capacity, batteries containing these high-energy materials need to be operated at higher voltages, such as voltages above about 4.2V. However, above about 4.3V, conventional electrolytes degrade and this leads to a significant deterioration of the cycle life. Further, the decomposition of the electrolyte at higher voltages can generate gas (such as CO2, O2, ethylene, H2) and acidic products, both of which can damage a battery. These effects are further enhanced in “high nickel” NMC compositions such as LiNi0.6Mn0.2Co0.2O2 or LiNi0.8Mn0.1Co0.1O2 or others which can provide higher capacities due to the electrochemistry of the nickel.
Many of these same challenges occur when a battery is operated at high temperature. That is, conventional electrolytes can decompose by oxidation or may be degraded by reduction at high temperature analogous to the way these mechanisms affect the electrolytes at high voltage. Other parasitic reactions can also occur at elevated temperature.
As disclosed herein, these challenges and others are addressed in high energy lithium ion secondary batteries including cathode active materials that are capable of operation at high voltage and/or high temperature.